Pulsed power-based dry fire protection for electric water heaters

ABSTRACT

A water heater having an electric heating element therein is provided with apparatus for preventing dry firing of the heating element. The apparatus is operative to (1) power the heating element with electrical test pulses having first predetermined durations and being separated by rest periods of second predetermined durations during which the heating element is depowered, (2) determine the average electrical current flow through the element during each of the test pulses, and (3) preclude energization of the heating element if the average current flow therethrough during an electrical test pulse subsequent to the first test pulse is less by a predetermined magnitude than the average electrical current flow through the heating element during the first electrical test pulse.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. patent application Ser.No. 13/938,964 entitled “Pulsed Power-Based Dry Fire Protection forElectric Water Heaters,” filed Jul. 10, 2013, which claims the benefitof the filing date of provisional U.S. Patent Application No. 61/678,704filed Aug. 2, 2012, the entire disclosures of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to dry fire protection forelectric water heaters and, in a representative embodiment thereof, moreparticularly provides pulsed electrical power-based dry fire protectionapparatus for liquid heating apparatus such as electric water heaters.

An electric water heater, like its fuel-fired counterparts, is typicallysold without water in it and is filled with water after being moved toand installed in its intended operation location. The possibility existsthat the water heater can be “dry fired”—i.e., have its electricresistance type heating element(s) energized before the storage tankportion of the water heater is filled with water to immerse the heatingelements(s) projecting into its interior. When such dry firing occurs,each dry fired electric heating element typically burns out, resultingin a return of the unit to the manufacturer, or a service call by arepair technician to perform on-site element replacement.

The cost of either repair procedure can be quite substantial, and isoften borne by the water heater manufacturer under its warranty policyfor the water heater. There is accordingly a need for reducing warrantycosts associated with dry firing of electric heating elements in a waterheater. It is to this need that the present application is primarilydirected.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a pulsed power-based dry fireprotection system operatively connected to an illustrative electricwater heater and embodying principles of the present invention;

FIG. 2 is a schematic diagram of a signal conditioning circuit portionof the FIG. 1 dry fire protection system;

FIG. 3 is a logic flow diagram illustrating operation of the dry fireprotection system;

FIG. 4 is a graphical depiction of the current flow through the FIG. 1electrical heating element when tested using the dry fire protectionsystem during wet fire conditions; and

FIG. 5 is a graphical depiction of the current flow through the FIG. 1electrical heating element when tested using the dry fire protectionsystem during dry fire conditions.

DETAILED DESCRIPTION

With initial reference to FIG. 1, in an illustrative embodiment thereofthe present invention provides specially designed dry fire protectionapparatus 10 which may be operatively associated with the power circuit12 of an illustrative electric water heater 14 (or other type of liquidheater using at least one electric liquid heating element) to preventdry fire damage thereto. Water heater 14 includes the usual tank 16within which a quantity of water to be heated and stored for on-demanddelivery to various plumbing fixtures operatively connected thereto.Such heated water is discharged from the tank 16 through a tank outletfitting 18, and automatically replaced with cold supply water, from asuitable pressurized source thereof, via a tank inlet fitting 20.

The power circuit 12 includes a grounded source 22 of high voltage ACelectrical power which is operable, via leads L1 and L2, to selectivelydeliver electrical power to at least one electric heating element 24disposed in the tank 16 and selectively energizable, by a thermostat 26connected in the power circuit 12 as shown, to maintain the tank waterat a predetermined set point temperature. In a conventional manner, thethermostat 26 continuously senses the tank water temperature, asschematically depicted by the lead 28, and is provided with a normallyopen switch portion 30 which is closed in response to sensing by thethermostat 26 of a tank water temperature below the set pointtemperature, and then permitted to re-open when the set pointtemperature is reached and the thermostat-sensed water heating demand issatisfied.

The dry fire protection apparatus or circuit 10 includes a relay 32connected in lead L1 in series with the thermostat 26, a signalconditioning circuit 34, a pre-programmed microcontroller 36 and analarm 38. Signal conditioning circuit 34 is coupled to electrical powerlead L1, representatively at location A thereon, by a step-down currenttransformer/sensor portion 40 of the circuit 34, the transformer/sensor40 illustratively having a 1000:1 winding ratio. As subsequentlydescribed herein, the signal conditioning circuit 34 outputs to themicrocontroller 36 a low voltage DC electrical signal 42, the voltage ofwhich is indicative of the alternating current passing through the waterheater heating element 24. The dry fire protection system 10 furtherincludes an AC/DC converter 43 and a relay drive circuit 45. AC/DCconverter 43 receives AC voltage from the electrical power source 22 viapower lead 44 and outputs DC electrical power to the microcontroller 36,the signal conditioning circuit 34, and the relay drive circuit 45respectively via power leads 47, 49 and 51. Microcontroller 36 outputscontrol signals 53,55 respectively to the alarm 38 and the relay drivecircuit 45, with relay drive circuit 45 outputting a control signal 46to the relay 32 to selectively open and close the relay 32.

Turning now to FIG. 2, in the representatively illustrated embodimentthereof, in addition to the current transformer 40, the signalconditioning circuit 34 comprises, in a downward sequence from thetransformer 40, an AC/DC rectifier section 50, a voltage limiting diode52, a filter section 54, and an op/amp buffer circuit 56. As indicatedin FIG. 2 and previously described herein, the signal conditioningcircuit 34 is operative to output to the microcontroller 36 (see FIG. 1)the low DC voltage signal 42 (representatively up to 3.3V DC) which isindicative of the alternating current passing through the water heaterheating element 24. Illustratively, the 3.3V DC conditioning circuitoutput signal 42 is digitized in such a manner that the 3.3 voltsrepresents 1024 steps or counts, with each step or count representing aspecific amount of current passing through the electric heating element24. As will be readily appreciated by those of skill in this art, signalconditioning circuits of other constructions could alternatively beutilized without departing from principles of the present invention.

The microcontroller 36 is programmed to utilize its electricalcurrent-representative input signal 42 (see FIGS. 1 and 2) to preventdry firing of the electric heating element 24 using the representativelogic shown in the schematic flow diagram of FIG. 3 which will now bedescribed. In response to an initial start-up of the water heater 14 atstep 60 (and the presence of a heating demand condition which closes thethermostat switch 30), a counter is set to zero at step 62, and themicrocontroller 36, at step 64, momentarily closes the relay 32 andcauses the electric heating element 24 to be energized with an AC testvoltage for a predetermined short pulse period (illustratively onesecond) and increments the counter from zero to one.

Next, at step 66, the electrical test current flowing through theheating element 24 (represented by the input signal 42 to themicrocontroller 36) is measured and an average current flow through theheating element 24 for the time (one second) the element was momentarilyenergized is calculated. At step 68 the heating element 24 is thende-energized for a predetermined rest period time (representatively forten seconds) by permitting the relay 32 to return to its normally openposition.

A transfer is then made to step 70 at which a query is made as towhether the counter value is “1”. If it is, a transfer is made to step72 at which the previously determined average current flow through theelement 24 is saved as a “threshold” value and a transfer is made tostep 78. If at step 70 the counter value is not “1”, a transfer is madeto step 74.

At step 74 a query is made as to whether the “average” element testcurrent value (previously calculated at step 66) is less than the“threshold” test current value minus a predetermined current value(representatively 132 mA in “counts”). If the answer is yes, a dry firecondition has been detected and a transfer is made to step 76 at whichthe alarm 38 is activated and the relay 32 is kept in its normally openposition to preclude energization voltage input to the heating element24. If the answer to the query at step 74 is “no”, a transfer is made tostep 78 at which a query is made as to whether the counter value isgreater than five. If the answer is “yes” a transfer is made to step 80at which normal water heater operation is permitted to satisfy thethermostat-initiated water heating demand.

If the query answer at step 78 is “no”, a transfer is made from step 78back to step 64, whereupon steps 64-74 are repeated. If the query answerat step 74 is “no” six times in a row in a given dry fire test, and thequery answer at step 78 then becomes “yes”, a transfer is made from step78 to step 80 at which the system has determined that a dry firecondition does not exist, and permits the operative energization of theelectric heating element 24 to satisfy the water heating demand.However, if at any time in a given dry fire test the query answer atstep 74 is “yes”, operative energization of the heating element 24 isprecluded at step 76 and the dry fire test is concluded.

As can be seen from the FIG. 3 logic flow chart, in an illustratedrepresentative embodiment thereof the present invention provides amethod for protecting a liquid heating apparatus electric heatingelement from being dry fired that, from a broad perspective, comprisesmeasuring the current flowing through the heating element in apredetermined number of pulsed increments spaced apart by predeterminedrest periods, determining the average current flow through the heatingelement during each current pulse, and preventing operationalenergization of the heating element if any of the determined averagecurrent flows through the element is less by a predetermined magnitudethan a “threshold” value equal to the first pulse average current flowthrough the heating element.

To illustrate this current average-to-threshold current averagecomparison to determine when a dry fire condition exists, reference isnow made to the graphs in FIGS. 4 and 5. FIG. 4 illustrates a non-dryfire test result in which each of the measured element current averagesis above the threshold value, while in FIG. 5 the last three measuredaverage element current flows are each below such threshold value to anextent indicating a dry fire condition.

As can be seen from the foregoing, dry fire testing of the element 24provided by the present invention may be advantageously carried outwithout subjecting the element 24 to substantial sustained periods oftest firing which are typically necessary when, for example, elementtemperature measurement is necessary to determine whether a dry firingcondition exists. Moreover, the dry fire protection system is of asimple, reliable construction that may be readily associated with aheating element power circuit.

The foregoing detailed description is to be clearly understood as beinggiven by way of illustration and example only, the spirit and scope ofthe present invention being limited solely by the appended claims.

What is claimed is:
 1. A method of preventing the dry firing of anelectric heating element, comprising the steps of: powering the electricheating element with a plurality of electrical test pulses having firstpredetermined durations and being separated by rest periods of secondpredetermined durations during which the electrical heating element isdepowered; determining an average of a representative valuecorresponding to an electrical current flow through the electric heatingelement during each of the plurality of electrical test pulses; andprecluding the operative energization of the electric heating element ifthe average of the representative value corresponding to the electricalcurrent flow through the electric heating element during an electricaltest pulse subsequent to the first electrical test pulse is less by apredetermined amount than the average of the representative valuecorresponding to the electrical current flow through the electricalheating element during the first electrical test pulse.
 2. The method ofclaim 1, further comprising disposing the electric heating element in anelectric water heater tank operative to store a quantity of water to beheated.
 3. The method of claim 1 wherein: the first predetermineddurations are each approximately one second, and the secondpredetermined durations are each approximately ten seconds.
 4. Themethod of claim 1 wherein: the electric heating element is incorporatedin a liquid heating apparatus having an AC electrical power circuit inwhich the electric heating element and a relay are operativelyconnected, and the powering step includes the step of alternatelyopening and closing the relay.
 5. The method of claim 1 wherein: thepowering step is performed using AC electrical power, and thedetermining step includes the step of utilizing the AC electrical powerto create a DC electrical signal indicative of the representative valuecorresponding to the electrical current flow through the electricheating element during each of the plurality of electrical test pulses.6. The method of claim 1 wherein: the AC electrical power is suppliedfrom an AC electrical power circuit, and the utilizing step includes thestep of coupling to the AC electrical power circuit to a signalconditioning circuit operative to output the DC electrical signal. 7.The method of claim 6 wherein: the coupling step is performed using astep-down current transformer.
 8. The method of claim 6, comprising thestep of using a pre-programmed microcontroller to receive the DCelectrical signal and responsively perform the precluding step.
 9. Themethod of claim 8 wherein: the pre-programmed microcontroller has apower input and a power output, and the step of using a pre-programmedmicrocontroller includes the step of operatively connecting the powerinput and the power output to the AC electrical power circuit.
 10. Themethod of claim 9 wherein: the AC electrical power circuit has anormally open relay therein, and the operatively connecting stepincludes the step of operatively connecting the power output of thepre-programmed microcontroller to the normally open relay.
 11. Themethod of claim 1, comprising the step of permitting the AC electricalpower circuit to continuously energize the electric heating element, tosatisfy a liquid heating demand of the liquid heating apparatus, whennone of a predetermined number of the electrical test pulses subsequentto the first electrical test pulse has created in the electric heatingelement an average current flow less by the predetermined amount thanthe representative value corresponding to the electrical current flowthrough the electric heating element during the first electrical testpulse.
 12. The method of claim 1, comprising the step of converting ACelectrical power received from the AC electrical power circuit to a DCelectrical output signal indicative of an average electrical currentflow through the electric heating element during each of the pluralityof electrical test pulses, and utilizing the DC electrical output signalin determining the average electrical current flow through the electricheating element during each of the plurality of electrical test pulses.13. The method of claim 12, comprising the step of coupling a signalconditioning circuit to the AC electrical power circuit by a step-downcurrent transformer, the signal conditioning circuit being operative togenerate the DC electrical output signal.
 14. A method of testing anelectric heating element for a dry fire condition, the method comprisingthe steps of: coupling a dry fire protection circuit to the electricheating element; powering the electric heating element with a pluralityof electrical test pulses having first time periods and being separatedby second time periods; determining a first average of a representativevalue corresponding to a first electrical current flow through theelectric heating element during one of the electrical test pulses;determining a second average of a representative value corresponding toa second electrical current flow through the electric heating elementduring another one of the electrical test pulses; and precluding theoperative energization of the electric heating element if the firstaverage is less than the second average by a predetermined amount. 15.The method of claim 14, further comprising depowering the electricheating element during the second time periods.
 16. The method of claim14, further comprising disposing the electric heating element in anelectric water heater tank operative to store a quantity of water to beheated.
 17. The method of claim 14 wherein: the electric heating elementis incorporated in a liquid heating apparatus having an AC electricalpower circuit in which the electric heating element and a relay areoperatively connected, and the powering step includes the step ofalternately opening and closing the relay.
 18. The method of claim 14wherein: the powering step is performed using AC electrical power, andthe determining steps include the step of utilizing the AC electricalpower to create a DC electrical signal indicative of the first andsecond averages of the representative values corresponding to the firstand second electrical current flows through the electric heating elementduring the electrical test pulses.
 19. The method of claim 14 wherein:the AC electrical power is supplied from an AC electrical power circuit,and the utilizing step includes the step of coupling to the ACelectrical power circuit to a signal conditioning circuit operative tooutput the DC electrical signal.
 20. The method of claim 19 wherein: thecoupling step is performed using a step-down current transformer.